OBJECTIVE: In virally-induced sarcomas there are four general classes of antigens potentially recognized by the host; these are 1) virion structural proteins, 2) modified precursors to virion proteins, 3) viral gene products not related to virion structural components (i.e., "sarc" gene products), and 4) host proteins modified by viral gene expression. The basic goal of this proposal is to define in molecular terms the antigens recognized by a sarcoma-bearing host. As a part of this work, we will take advantage of the genetically well-characterized avian system to identify the "sarc" gene product from a series of sarcoma viruses, and to establish the extent of diversity within this family of molecules.